1. Field of the Invention
The present invention relates to a three-dimensional measurement apparatus for measuring the position and orientation of a measurement object and a control method therefor.
2. Description of the Related Art
Three-dimensional measurement techniques used in imaging apparatuses include active stereo methods that enable corresponding points to be clearly determined by projecting structured light onto a measurement object. Active stereo methods are little affected by whether or not the object has a textured surface. With a coded pattern light projection method constituting one such active stereo method, partitioning into 2n space codes is made possible by performing light projection and imaging n times, enabling accurate measurement to be performed at high speed. As for the coding method, the Gray code disclosed in James R. Bitner, Gideon Erlich, and Edward M. Reingold, “Efficient generation of the binary-reflected Gray code and its applications”, Communications of the ACM, 19(9), pp. 517-521, 1976 (hereinafter, Document 1) is often used.
On the other hand, a method for estimating position and orientation, in the case where a geometric model of the measurement object surface is known and a plurality of distances of the measurement object surface from the imaging apparatus are derived as a point set, so as to reduce the difference in distance between the point set and the closest point, is described in P. J. Besl and N. D. McKay, “A method for registration of 3-D shapes”, IEEE Transaction on Pattern Analysis and Machine Intelligence, 14(2): 239-256, Feb. 1992 (hereinafter, Document 2). This is widely used as an iterative closest point (ICP) method for distance measurement and alignment of geometric models. Similarly, Y. Chen and G. Medioni, “Object modeling by registration of multiple range images”, Proceedings of the IEEE International Conference on Robotics and Automation, 1991, vol. 3, pp. 2724-2729, April 1991 (hereinafter, Document 3) involves calculating the distance between a set of measurement points and the surface of a geometric model, and enables the position and orientation of a measurement object to be estimated so as to minimize the error between the geometric model of the measurement object and the set of measurement points.
When the measurement object moves while projecting and imaging projection patterns, three-dimensional measurement fails because of not being able to find correspondences between patterns. Thus, with three-dimensional measurement of a moving measurement object, the time taken to perform projection and imaging for distance measurement needs to be shortened as much as possible. Measuring the orientation of a person is an example of the measurement of a moving measurement object. Mounting a measurement apparatus on a robot arm with visual feedback installed in the robot is an example of a projection/imaging entity that moves. Generally, three-dimensional measurement of a measurement object is possible by performing projection and imaging faster than the movement of the object. Faster projection and imaging of patterns for three-dimensional measurement of a moving measurement object is thus sought.